Black King Bar is a situational item recommended for any hero in need of protection from spells. A useful time to get BKB is when you're a high priority target and need to stop the enemy team from focusing you down with too many spells.

Because Avatar's duration and cooldown are tied to the user, buying a new one will not reset the values. The unit is stuck with what they have permanently.

An important example is Lone Druid. If Lone Druid has the minimum cooldown and duration, then gives his BKB to his Spirit Bear which hasn't used it yet, the Spirit Bear will have the maximum cooldown and duration due to not having used BKB yet, since the values are tied to the unit and not the item.